The present invention relates to an air bearing slider for use in a data storage device such as a disc drive. More particularly it relates to an air bearing slider capable of operating at ultra-low flying heights.
Air bearing sliders have been extensively used in magnetic disc drives to appropriately position a transducing head above a rotating disc. In a disc drive, each transducer “flies” just a few nanometers above a rotating disc surface. The transducer is mounted in a slider assembly which has a contoured surface. An air bearing force is produced by pressurization of the air as it flows between the disc and slider and is a consequence of the slider contour and relative motion of the two surfaces. The air force prevents unintentional contact between the transducer and the disc. The air bearing also provides a very narrow clearance between the slider transducer and the rotating disc. This allows a high density of magnetic data to be transferred and reduces wear and damage.
In most high capacity storage applications, when the disc is at rest, the air bearing slider is in contact with the disc. During operation, the disc rotates at high speeds, which generates a wind of air immediately adjacent to the flat surface of the disc. This wind acts upon a lower air bearing surface of the slider and generates a lift force directing the slider away from the disc and against a load beam causing the slider to fly at an ultra-low height above the disc.
In negative pressure sliders, the wind also acts upon a portion of the air bearing surface of the slider to generate a suction force. The suction force counteracts the lift force by pulling the slider back toward the surface of the disc. A slider is typically mounted on a gimbal and load beam assembly which biases the slider toward the rotating disc, providing a pre-load force opposite to the lift force acting on the air bearing surface of the slider. For the slider to maintain the ultra-low flying height above the surface of the disc, the lift force must be balanced with the pre-load and suction forces.
As disc storage systems are designed for greater and greater storage capacities, the density of concentric data tracks on discs is increasing (that is, the size of data tracks and radial spacing between data tracks is decreasing), requiring that the air bearing gap between the transducing head carried by the slider and the rotating disc be reduced. One aspect of achieving higher data storage densities in discs is operating the air bearing slider at ultra-low flying heights.
However, shrinking the air bearing gap and operating the slider at ultra-low flying heights has become a source of intermittent contact between the transducing head and the disc. Furthermore, when a disc drive is subjected to a mechanical shock of sufficient amplitude, the slider may overcome the biasing pre-load force of the load beam assembly and further lift away from or off the disc. Damage to the disc may occur when the slider returns to the disc and impacts the disc under the biasing force of the load beam. Such contact can result in catastrophic head-disc interface failure. Damage to the disc may include lost or corrupted data or, in a fatal disc crash, render the disc drive inoperable. Contact resulting in catastrophic failure is more likely to occur in ultra-low flying height systems. Additionally, intermittent contact induces vibrations detrimental to the reading and writing capabilities of the transducing head.
For the disc drive to function properly, the slider must maintain the proper fly height and provide adequate contact stiffness to assure that the slider does not contact the disc during operation. Also, the air bearing slider must have enhanced take-off performance at start up to limit contact between the slider and the disc. Such contact would cause damage to the slider during take-off and landing of the slider.
Air bearing sliders typically possess three primary degrees of movement, which are vertical motion, pitch, and roll rotation. The movement is relative to the gimbal and load beam associated with three applied forces upon the slider defined as pre-load, suction, and lift force. Steady state fly attitude for the slider is achieved when the three applied forces balance each other. A typical air bearing slider has a taper or step at its leading edge to provide for fast pressure buildup during takeoff of the slider from a resting position to a flying altitude above the disc. Air bearing sliders have a trailing edge at which thin film transducers are deposited. Typically, the air bearing surface includes longitudinal rails or pads extending from the leading edge taper toward the trailing edge. The rail design determines the pressure generated by the slider. The pressure distribution underneath the slider determines the flying characteristics, including flying height and pitch and roll of the slider relative to a rotating magnetic disc. Other characteristics that are affected by the configuration of the air bearing surface of a slider are takeoff velocity, air bearing stiffness, and track seek performance.
Flying height is one of the most critical parameters of magnetic recording. As the average flying height of the slider decreases, the transducer achieves greater resolution between the individual data bit locations on the disc. Therefore, it is desirable to have the transducers fly as close to the disc as possible.
In a conventional air bearing slider, the slider body is formed from a substrate wafer of conductive ceramic material. On this substrate, a thin film of insulating material is deposited, and a metallic transducer is built therein, by a process such as sputtering. The transducer, which typically includes a writer portion for storing magnetically-encoded information on a magnetic media and a reader portion for retrieving that magnetically-encoded information from the magnetic media, is formed of multiple layers successively stacked upon the substrate. The volume of the transducer is typically much smaller than the volume of the substrate.
The wafer with transducers formed thereon is then cut into bars, and a cut edge of each bar is lapped to form an air bearing surface. The layers of the transducer, which include both metallic and insulating layers, all have different mechanical and chemical properties than the substrate. These differences in properties affect several aspects of the transducer. First, the different materials of the slider will be lapped at different rates. Because of the difference in hardness or lapping durability of the wafer substrate material, the thin film insulating material, and the transducers, the lapping operation results in differential recession of the materials at the air bearing surface.
Thus, when an air bearing surface of a slider is lapped during its fabrication, differing amounts of the different materials will be removed—resulting in the slider having a uneven air bearing surface. The recession of a particular component is defined as the distance between the air bearing surface of the ceramic substrate and the air bearing surface of that component. Commonly, a greater amount of the metallic layers of the transducer will be removed during the lapping process than will be removed from the slider body substrate. Thus, this lapping process results in a Pole Tip Recession (PTR) of the metallic layers of the transducer with respect to the slider body substrate.
Additionally, the insulating material will often recede at an even greater rate than the transducer, leading to material recession that results in a discernable offset at the interface of the insulating material and the slider body substrate material. The offset prevents the transducer from flying as close to the surface to the magnetic disc as would otherwise be possible.
Further, the differing mechanical and chemical properties of the substrate and transducer layers further affect the air bearing surface during operation of the transducing head. As the magnetic data storage and retrieval system is operated, the transducing head is subjected to increasing temperatures within the magnetic data storage and retrieval system. In addition, a temperature of the transducing head itself, or a part thereof, may be significantly higher than the temperature within the magnetic data storage and retrieval system due to heat dissipation caused by electrical currents in the transducer.
The coefficient of thermal expansion (CTE) is a measure of the change in length for a unit length of material for an incremental change in temperature. The CTE of materials used in forming the substrate is typically much smaller than the CTE of materials used in forming the metallic layers of the transducer. Due to the larger CTE of the transducer's metallic layers, those layers will tend to expand a greater amount than will the substrate. Thus, when the transducing head is subjected to higher operating temperatures, the metallic layers tend to protrude closer to the magnetic disc than the substrate; thereby affecting the PTR of the transducer. This change in PTR caused by temperature is referred to as the Thermal PTR (T-PTR).
During operation of the magnetic data storage and retrieval system, the transducing head is positioned in close proximity to the magnetic media. A distance between the transducer and the media is preferably small enough to allow for writing to and reading from a magnetic medium having a large a real density, and great enough to prevent contact between the magnetic media and the transducer. Performance of the transducer depends primarily on this distance.
Thus, a need exists for an air bearing slider design which achieves a constant, ultra-low transducer flying height, despite the obstacles of differential mechanical and thermal recession. This should be accomplished while minimizing the chance of contact between the transducer and the magnetic disc surface.